1. Field of the Invention
The present invention relates to a pollution control method for removing sulfur oxides, mercury vapor, and fine particulate matters from industrial flue gases, such as coal-fired power plant flue gas.
2. Description of the Related Art
Coal-fired power generation plants, municipal waste incinerators, and oil refinery plants generate huge amounts of flue gases that contain substantial varieties and quantities of environmental pollutants, such as sulfur oxides (SO2, and SO3), nitrogen oxides (NO, NO2), mercury (Hg) vapor, and particulate matters (PM). In the United States, burning coal alone generates about 27 million tons of SO2 and 45 tons of Hg each year.
The destructive effects of various coal-burning pollutants on human health and on the ecosystem were recognized a long time ago. For example, SOx and NOx have been linked to the outbreak of respiratory diseases in the affected areas. They also form acid rains, which damage forests, fisheries, and architectures. As for Hg, it is a potent toxin to the nervous system. Exposure to mercury can affect the brain, spinal cord, and other vital organs. It is particularly dangerous to developing fetuses and young children. Relatively less attention is paid to the particulate matters (PM). However, fine particulates, especially those of less than 2.5-micrometer size (PM2.5), cause great health problems on human beings. PM2.5 is typically loaded with various toxic chemicals such as sulfates, nitrates, and heavy metals. PM2.5 is found to trigger heart attacks, damage lungs and kill thousands of people every year.
The typical methods of removing pollutants from industrial flue gases are designed to remove individual pollutants. For example, the prevailing technology for flue gas desulfurization (FGD), or SO2 removal, is the limestone based wet scrubber or dry scrubber, which uses alkali limestone to neutralize and remove SOx. The prevailing technology for flue gas NOx removal is the selective catalytic reduction (SCR), which uses ammonia or urea to catalytically convert NOx into nitrogen, oxygen, and water. These technologies are typically very complicated and expensive.
In U.S. Pat. No. 6,132,692, a process for reducing multiple pollutants (particles, Hg, NOx, and SO2) is disclosed. In this process, an electrical barrier discharge reactor produces the HgO and acids HNO3 and H2SO4, a wet electrostatic precipitator (ESP) collects the HgO, acids, and particulates. The collected pollutants are then drained from the wet ESP for further processing. However, the SO2 and NOx removal efficiencies of this process are limited, while the system is expensive, energy input is very high, and the collected acid solution may need treatment as liquid waste.
Activated carbon based flue gas purification technology is frequently studied and has enjoyed some commercial success. The technology can potentially remove both SOx and mercury vapor simultaneously. In U.S. Pat. No. 3,486,852, an adsorbing process and apparatus for the removal of SO2 from industrial waste gases is disclosed. The adsorbing units consist of an adsorbing zone, two regenerating (or washing) zones and a drying zone. The washing liquid (water) from the washing zone can be neutralized with an alkaline compound in a neutralization tank or passed to an acid concentrator. In U.S. Pat. No. 4,164,555, a pollution control system, along with the method in which SO2 in flue gases are adsorbed by activated char in a gas-solid contacting device, is disclosed. The saturated char is regenerated in an integrated desorption-reduction vessel; to which crushed coal and combustion-supporting air are supplied. The regeneration process generates SO2 and CO2 and consumes a portion of the char. The generated SO2 can be fed into a sulfuric acid manufacturing plant.
All the activated carbon based flue gas purification technologies require a carbon regeneration process, because the activated carbon will be saturated by the adsorbed SO2 or the converted SO3 and/or H2SO4. The regeneration process requires either high temperature degassing or water washing. The disadvantages of a regenerating process are: 1) it consumes activated carbon; 2) it generates secondary pollution, such as low concentration acid solution; and 3) it makes the overall system complicated and expensive.
Therefore, there is a need to provide a simple system that can simultaneously remove multiple flue gas pollutants such as SOx, Hg vapor, and PM2.5 with low cost. It is desirable that the system is simple, does not generate secondary pollutions, and has the capability of producing a useful end product. More specifically, an activated carbon based system without a costly and complicated regeneration process is desirable.